In a vehicle engine which is running idle, when load increases, the engine rotation speed falls considerably. To prevent this drop of engine rotation speed, a controller is known in the art which performs feedback control such that the idle rotation speed coincides with a target idle engine rotation speed via intake air volume control.
However, in this feedback control of idle engine rotation speed via intake air volume control, there is a response delay from when intake air volume is increased to engine torque increase, and due to this delay, it may occur that the drop in rotation speed when there is a sudden load change cannot be corrected in time.
To resolve this problem, Tokkai Sho 57-83665 published by the Japanese Patent Office in 1982 discloses how idle engine speed is controlled to a target idle engine speed in a short time by applying ignition timing control which has a small response delay, in conjunction with intake air volume control.
However even in this control system, when feedback control of idle engine rotation speed is started, the feedback correction amount of intake air volume and ignition timing becomes large if the engine rotation speed largely exceeds a target rotation speed NSET. In particular, when integral control is applied to feedback control, the integral part of the feedback correction amount of the intake air volume largely increases in a negative direction as shown in FIG. 21B so the intake air volume temporarily suffers a serious decrease. As a result, the engine rotation speed drops below the target idle engine rotation speed as shown in FIG. 21A.
This drop in rotation speed is particularly marked after the load suddenly decreases due to shift down of an automatic transmission, the engine rotation speed suddenly increases temporarily, and the difference between the engine rotation speed and target idle engine rotation speed becomes large, as shown in FIGS. 22A and 22B.
Tokkai Hei 2-70955 published by the Japanese Patent Office published in 1990 suggests that to deal with this phenomenon, when a shift-down of the automatic transmission occurs during feedback control of the idle rotation speed, feedback control is stopped for a predetermined time.
Further, to suppress hunting of the engine rotation speed, the engine rotation speed is increased by advancing the ignition timing by a constant amount when feedback control has stopped so as to increase output power.
However in this case, if load fluctuations occur and the engine rotation speed largely fluctuates when feedback control has stopped, the rotation speed may still fall below the target rotation speed even when the ignition timing is advanced as feedback control is not active.
If for example the intake air volume is largely increased when feedback control has stopped so as to prevent drop of rotation speed, the engine rotation speed increases when load fluctuations do not occur so that drivability and fuel cost-performance are impaired.